This is a special kind of heat engine. It is probably the most useful kind of heat engine known to man, because it effectively converts heat energy into useful mechanical energy in a small, efficient power plant.
Examples of Internal combustion (IC) engines include Petrol engines, two-stroke engines, diesel engines, Wankel rotary engines, rocket motors and gas turbines.
Steam engines, fuel cells, batteries and electric motors are not IC engines.
There are at least three ways to tackle this subject,
- Historical review
- Thermodynamic theory
- Practical mechanics
All have their own insights to offer.
Early in the Industrial revolution, James Watt , Richard Trevethick and Thomas Newcomen started developing their steam engines. Although they were not internal combustion engines, these were the first practical heat engines.
A steam engine uses a pressurised gas (steam) to push a piston in and out of a cylinder. The high pressure in the gas is generated by boiling water in a confined space, to produce steam at high pressure and temperature. At about the same time as the engineers were inventing their machines, in the late 1600 and early 1700s, Robert Boyle was working out what happens to gases as they are compressed. Gradually, as the machine designers became more confident and the mines and cotton factories required engines with more power, the engineers got thinking about ways to get more mechanical energy out of their engines.
They discovered that the maximum energy you can get out of the steam engine is linked to the maximum temperature of the raw steam. This realisation marked the beginning of the science of thermodynamics (Literal translation: motion from heat). And it turns out that this is a fundamental property of all heat engines. The thermal efficiency (so-called) is governed by the temperature difference between the hottest and coldest parts of the energy cycle, and the absolute temperature of the hottest part of the cycle. No need to worry exactly what that means, but to understand this stuff, you need to realise that the hotter you can make your heat engine, the more power you can get out of it.
Move on a couple of centuries, and we come to Rudolf Diesel in Germany in the late 1800s. By this time, people had pretty much worked out the best way to build steam engines, and had made them as efficient and powerful as they could be. The scientists had worked out the numbers and knew that temperature and pressure were important to deliver as much power as possible from a small volume. The problem with steam was that a powerful engine required steam at high temperature and pressure, and that meant large, heavy pipes and pressure vessels to contain the steam, and deliver it to the cylinders.
Diesel got to thinking about ways to generate high pressures inside the cylinder. Instead of generating the high pressure steam outside the cylinder, and pumping it into the cylinder to move the piston, Diesel wanted to introduce the fuel and oxygen into the cylinder, burn it very quickly, to generate a big increase in temperature and volume, which would push the piston out quickly, without the need for a huge pressure vessel to contain the steam. Radical!
In 1893, he wrote a famous patent
, which was published in 1894, and by 1897, had built a practical engine based on the idea of internal combustion
He was so smart, he used the piston first to compress the gas, and raise it above a critical temperature. As the gas got to its hottest, he squirted fuel into the cylinder and, with the help of a glowing electric wire, the fuel/air mixture ignited. Bang!
The mixture exploded, and rammed the piston down the cylinder, driving a huge flywheel. The inertia of the flywheel was more than enough to compress the next charge of gas. The energy left over could be used to drive a pulley or a generator. This was the first engine to use internal combustion, it had a thermal efficiency of around 25 percent, substantially higher than competing engines, which were running around 20 percent.
It was truly a radical invention, but has become modernised and refined over the subsequent century into the engines used in almost all cars and trucks. The internal combustion engine is compact, reasonably efficient and very reliable. Modern diesel engines can deliver efficiencies around 40 or 45 percent, with excellent reliability. Good control systems and direct fuel injection have removed the need for the coil, or glow-plug. A compression ratio of 15 or 20:1 ensures the air temperature is easily high enough to ignite the fuel as it is injected.
Once Diesel had proved that internal combustion could work, the age of steam was over. Within 20 years, steam engines became old technology. Internal combustion engines caused a revolution in transport, as these small, light engines offered power-to-weight ratios far in excess of the most sophisticated stream engines. First motor cars came along, followed after just a few years by the first powered-aircraft, and then ships and finally trains were converted to energy derived from internal combustion engines.
The last bastions of steam power are the railways in India, China and other developing parts of the world, where steam engines are still reliable and easy to maintain, and offer a power source dependent on cheap, easily-mined coal, rather than expensive, refined petroleum.
While most people think of the IC engine as a discontinuous engine based on a series of discrete explosions taking place in an arrangement of pistons and cylinders, there is one very important class of IC engine which is based on a continuous burning process.
That is the gas turbine, most commonly used as the power plant in large machines, such as aircraft, ships and locomotives, as well as in commercial power stations.
The gas turbine is an internal combustion engine, because the fuel is burned inside the engine. Once the fuel/air mixture has been burned, it is much hotter and at a higher pressure than the ingredients. So the exhaust gases press on the front half of the combustion chamber, and stream out at the back, causing forward motion. On their way out, they pass through a turbine, which is attached to a compressor at the front of the engine, and this sucks in air, compresses it and pumps it into the combustion chamber.
The temperatures in a gas turbine are the highest commonly reached in any heat engine here on earth, which makes them very efficient by the standards of other engines, and the prodigious power output of a large gas turbine is down to this high thermal efficiency, combined with a simple and efficient process. However, the sustained high running temperature and the large forces involved do test the materials from which the machine is built, to the limit.
IC engines are heat engines. Their maximum efficiency is governed by the efficiency of the idealised Carnot cycle. Beyond that, the engine has its own mechanical inefficiency, which reduces the overall efficiency still further. Nevertheless, over the years, engineers have found ways to maximise the energy they extract from the heat (intercoolers, turbo chargers and so on) and have increased the operating efficiencies to very high levels.
These cycles are represented by lines drawn on charts which represent the characteristics of the working fluid. The graphs can show pressure, volume, entropy, enthalpy or a few other parameters, In every case, the working fluid (steam in a steam engine, air in most IC engines) moves through a cyclic path, from hot, high pressure to cooler lower pressure and back again. if the axes are chosen correctly, then the energy output corresponds to the area enclosed by the cyclic graph. The goal of the engine designer is to find a place on the graph where the operating cycle offers the best compromise between temperature differential and energy output.
The practical details
Internal combustion engines have come a long way since Diesel's 10-foot single cylinder prototype blew up.
Nowadays they are made in any number of sizes and configurations. A typical gas turbine used to spin a generator used in a power station has can deliver tens of megawatts of mechanical power. Few other mechanical systems can handle this prodigious amount of power. Nevertheless, the power-to-weight ratio of a gas turbine is not strikingly better than a diesel engine, for example. An industrial 27 MW turbine might weigh 80 tonnes, with another 20 tonnes for control gear and so on, and a further 80 tonnes for a generator, leading to a combined weight for the complete generating set, of around 180 tonnes. For the turbine alone, the power output is around 300 W per kg of weight. Contrast this with a modern diesel engine which might only deliver 500 kW or so, but has a weight of 1000 kg: a ratio of 500 W/kg These are both exceptionally good compared with a modern miniature steam engine of 2 kW output, which weighs 30 kg (66W/kg), even without the boiler.
Efficiency too has advanced in leaps and bounds. Newcomen's first atmospheric engine has a thermal efficiency of around 1 percent, compared with a modern automotive diesel engine which might reach 44 percent thermal efficiency. Engine manuafacturers suggest that large diesel engines used in trucks might reach 50 or 55 percent thermal efficiency in the next decade or so, as the use of turbo chargers and intercoolers advances, and as the science of burning, swirling fluids advances
Note This piece written, formatted and edited in Dann's E2 offline scratchpad